Oil and gas wells frequently require stimulation in order to recommence or improve a flow of hydrocarbon from the hydrocarbon bearing formation with which a well bore communicates. Well stimulation generally involves the pumping of fluid mixtures into the hydrocarbon formation at extreme pressures. The fluid mixtures frequently comprise acid solutions and/or abrasive proppants such as bauxite granules or sand. Wellheads generally comprise one or more valves, a casing spool, tubing spool, tubing hanger, blowout preventer, and related apparatus which is designed to contain and control well fluids at well pressures. Wellheads are not usually designed to withstand the abrasive effects of well stimulation proppants or the extreme pressures of well stimulation treatments. It is therefore necessary to provide a tool for isolating the wellhead from the caustic and/or abrasive fluids as well as the extreme fluid pressures used in well stimulation treatments. Many wellhead isolation tools have been invented for satisfying this requirement. The prior art considered most relevant to the present invention includes:
Canadian Patent 1,137,869--Surjaamadja
Canadian Patent Application 1,277,230--McLeod
Canadian Patent Application 1,281,280--McLeod
Canadian Patent Application 1,292,675--McLeod
Canadian Patent Application 2,055,656--McLeod
Wellhead isolation tools are alike in that each tool inserts a length of high pressure tubing, hereinafter referred to as a mandrel, through a vertical passage defined by the wellhead, the lower end of the tubing being sealed or packed off in the production tubing or casing of the well. Each tool also requires a mechanism to prevent wellbore gases from escaping to atmosphere while stroking the mandrel into our out of the wellhead. Each also requires a high pressure valve, traditionally connected to a top of the mandrel, to control pressure while the mandrel is seated and packed off in the well tubing or casing.
Well stimulation treatment is a potentially dangerous operation because of the extreme pressures used to inject fluid mixtures into a well bore. It is therefore desirable to provide a wellhead isolation tool which permits ready and easy access to the injection lines used for well stimulation. It is also desirable to provide a tool which locates the hook-up point for the injection lines as close to the wellhead as possible in order to minimize the stress of the racking forces sometimes exerted by fluid pressures in the injection lines. It also desirable to provide a wellhead isolation tool which can be extracted from the wellhead under any condition, including "screen out" in which a blockage occurs or the well's capacity for accepting abrasive proppants is exceeded and the entire apparatus, including the wellhead isolation tool, is packed with abrasives injected under extreme pressure. In addition, it is preferable to provide a wellhead isolation tool having a mandrel that is completely enclosed in a sealed bore. This feature is desirable for two reasons. First, an exposed stuffing box can leak or fail and discharge hydrocarbons to the atmosphere when a mandrel is being stroked in or out of a wellhead. Second, if large quantities of coarse abrasive are pumped in one session, the abrasives can wear a hole through the sidewall of a mandrel. If this occurs, an exposed mandrel cannot be safely stroked out of the well.
In a first generation of prior art wellhead isolation tools the high pressure valves used to control well stimulation fluid pressures are located too high above the wellhead and are therefore difficult to access. These first generation tools were also arranged so the apparatus used to stroke a mandrel through a wellhead was left on the wellhead during well stimulation treatments. This places strain on the wellhead and subjects the wellhead to excessive racking forces if "line jack" occurs during a stimulation treatment. "Line jack" is a high speed vibration or whipping of stimulation lines which occurs if a blockage develops while pumping slurries high in sand content or when a hydraulic valve malfunctions.
In a second generation of wellhead isolation tools, apparatus used to stroke the mandrel through a wellhead was designed to be removed from the wellhead during a stimulation treatment so that the high pressure valve was located in closer proximity to the wellhead, thereby making the valve more readily accessible and minimizing the racking forces exerted by line Jack. The disadvantage of the second generation tools is that they require that a portion of the tool be disassembled after the mandrel is stroked through the wellhead and reassembled before the mandrel can be stroked out of the wellhead. In the event of a blow-out or a fire during a stimulation treatment, it may be difficult or impossible to reassemble the tool in order to stroke out the mandrel so that the main control valve or a blow out preventer in the wellhead can be closed to bring the situation under control.
The known wellhead isolation tools fail to provide all of the desirable features required to minimize the hazards of well stimulation and maximize the ease and speed of preparing a well for stimulation treatment.